As a technique effective for removing particulate matter (exhaust particulate, hereinafter referred to as PM) contained in an exhaust gas of a diesel engine, a diesel particulate filter (hereinafter referred to as DPF) is known. DPF is a device for collecting PM employing a filter. DPF is provided on an exhaust passage of an engine, and is a device for collecting PM including soot exhausted from the engine with a filter and removing PM from the exhaust gas. A part of PM collected by the DPF will be combusted by a high-temperature exhaust gas exhausted from the engine during operation, but the remainder of the PM will be accumulated in the filter of the DPF. However, as DPF has a limited capacity for collecting PM, excessive progress of accumulation of PM may result in decline in the performance of collecting PM or reduction in the engine output. Thus, it is necessary for the DPF that a regeneration treatment for the filter where PM accumulated in the filter is combusted to regenerate the filter, is performed at an appropriate timing.
In order to get the appropriate timing to perform the regeneration treatment, it is necessary to accurately estimate the PM accumulation state in the DPF. If the PM accumulation state is underestimated, the time to perform the regeneration treatment may be delayed, and thus the engine output may be reduced due to excessive accumulation of PM, or the DPF may be damaged due to excessive heat during the regeneration treatment. On the other hand, if the PM accumulation state is overestimated, the frequency of performing the regeneration treatment may become high, and thus problems such as worsening of fuel consumption and oil dilution may occur.
A technique for accurately estimating the PM accumulation state in a DPF and performing the regeneration treatment at an appropriate timing is disclosed by the present applicant (Patent Document 1). According to the technique of Patent Document 1, the PM accumulation state of DPF is evaluated with a concept of “collection stages”, and the PM accumulation state is classified into six collection stages divided according to the PM accumulation state. In addition, as shown in FIG. 15, operation to be performed in the regeneration treatment is preliminarily determined depending upon the collection stage.
In the technique of Patent Document 1, the collection stage is determined based on the flowchart illustrated in FIG. 16. That is, according to the flowchart of FIG. 16, the current collection stage is determined in step S51, then the procedure move ahead to step S52. The “current collection stage” in step S51 is the collection stage which has been determined in the preceding cycle in the flow of FIG. 16 and which is stored in an ECU.
When the current collection stage X is determined as one of 1 to 6 (S52), four evaluation indices of (1) PM accumulation amount estimate, (2) cumulative operation time, (3) cumulative fuel consumption and (4) DPF corrected pressure difference are compared with the thresholds Qx, Tx, Qfx and dPx, respectively, in step S53. If a state where any one of the four indices exceeds the threshold is continued for a predetermined period of time, 1 is added to the collection stage number in step S55. If DPF is determined to be undergoing regeneration in step S54, the addition to the collection stage number is not performed.
According to Patent Document 1, the PM accumulation state is comprehensively evaluated by the four evaluation indices of (1) to (4), whereby it is possible to more accurately estimate the PM accumulation state compared with the case where the PM accumulation state is evaluated with a single index. Further, operation to be performed in the regeneration treatment is preliminarily determined depending upon the collection stage, whereby an appropriate regeneration treatment can be performed at an appropriate timing depending upon the PM accumulate state.